BOSTON (AP) — Boston Mayor Thomas Menino wasn't the smoothest of political figures, but during his nearly two decades in office he earned a reputation as one of the hardest working, from filling potholes to shaping the city's skyline.

Menino — beset with health problems — decided against seeking an unprecedented sixth term. He planned to make a formal announcement late Thursday afternoon at Boston's historic Faneuil Hall.

Menino said the decision was "very difficult" during an emotional, early morning news conference outside his home. He said he's lived the job "24-7" since becoming mayor in 1993.

"It's a changed city, and I'm glad to be a small part of this changed city," said the Democrat, crediting his staff and cabinet. He said the city is more tolerant, vital and with a younger population than when he took office.

A recent Boston Globe poll showed the 70-year-old Menino was viewed favorably by a wide margin of city residents, although less than half said they wanted him to run again.

Most political watchers assumed Menino could have cruised to another victory, however, despite health woes that had him hospitalized for nearly six weeks last year.

Menino was treated for a respiratory infection that developed during a vacation in Italy. While at the hospital he suffered complications including a compression fracture in a vertebra in his spine. Menino also was diagnosed with Type 2 diabetes.

After being discharged, Menino spent three months recuperating at the Parkman House, a city-owned mansion on Beacon Hill. He just recently moved back to his home.

Menino's long stewardship of the city came a critical moment in Boston's history when traditional urban ethnic enclaves began to give way to waves of new immigrants and younger professionals who began to stake their own claim on the urban landscape.

Lawrence DiCara, a former city councilor, said Menino's success stemmed from his attention to detail and — unlike many of his predecessors — his lack of interest in higher office.

"He kept his eye on the ball," DiCara said. "He was not interested in running for governor. He was not interested in running for Congress. He had one thing he wanted to do and that was being mayor of Boston."

Menino also worked to make Boston more fun and livable. Despite its famously narrow, twisting streets, Menino helped usher in a bicycle-sharing program and named a "bicycle czar" to negotiate conflicts between bicyclists and Boston drivers.

He also struggled to try to improve the Boston school system and wasn't shy about wearing his sympathies on his sleeve.

Last year Menino, a strong gay rights supporter, vowed to block Chick-fil-A from opening a restaurant in the city after the company's president spoke out publicly against gay marriage.

Menino was the city's first Italian-American mayor, breaking a nearly century-long domination of city politics by Irish-American mayors that began with John "Honey Fitz" Fitzgerald, the grandfather of President John F. Kennedy and Sens. Robert and Edward Kennedy, and would include the legendary James Michael Curley.

Menino also grew up in Hyde Park, far from the city's traditional political power bases. This, said DiCara — a onetime mayoral candidate himself — gave Menino empathy with residents who often felt ignored by City Hall.

"He grew up an Italian kid in an Irish city and he grew up in a neighborhood that no one came from," said DiCara.

Along with a drop in crime and strong city finances, Menino also could boast that in 20 years, no one in his administration was indicted or convicted, an "extraordinary" accomplishment, DiCara said, in a city with a sordid history of political corruption.

Menino's departure will create only the second open mayoral election in the last half century and the first since 1983, when White chose not to seek re-election.

City Councilor John Connolly had previously announced his intention to run for mayor, but the field appeared to be wide open with any number of current city councilors and Boston state legislators eyeing potential runs.

Among the possible candidates was Tito Jackson, an African-American city councilor and former aide to Gov. Deval Patrick. Jackson lauded Menino but declined to address his own future.

"Today is Tom Menino's day. After serving the city for more than twenty years, the mayor has earned all the love and gratitude we can give him," Jackson said in a statement.

DiCara said the demographic changes that have occurred in the city since Menino became mayor have helped open the door to any number of possible successors.

"The next mayor could be black, could be Spanish, could be gay," he said. "Anybody could be elected mayor of Boston."

Menino became acting mayor when his predecessor, Raymond Flynn, left office in 1993 after being named ambassador to the Vatican. Menino, then president of the City Council, was automatically elevated to the mayor's job.

The circumstances prompted some critics to label him the "accidental mayor," a charge the sometimes-thin-skinned Menino was quick to reject. But he was elected mayor in his own right in November 1993 and won re-election by wide margins in 1997, 2001, 2005 and 2009.

The city's previous longest serving mayor, the late Kevin White, was in office for four terms, from 1968 to 1984.

Menino's longevity also exceeded that of Curley, who also served four terms, but not consecutively.

Menino earned the nickname "Urban Mechanic" by focusing on the nuts and bolts of city management — fixing potholes, cleaning streets, even curbing the practice of saving a shoveled-out parking space by putting folding chairs or trash cans along the curb.

But he also tried to leave his imprint on the city's downtown area, sometimes weighing in on the design of new buildings that would reshape Boston's skyline.

The 2004 Democratic National Convention put Menino's political and negotiating skills to the test when the city's main police union threatened to picket over an unresolved contract.

More recently, Menino became a prominent voice in favor of stricter gun control measures.

He co-founded Mayors Against Illegal Guns with New York Mayor Michael Bloomberg and criticized the National Rifle Association's call for more armed guards at schools after the Connecticut school shooting in December.

During his years in office Menino also built an impressive political machine that handily defeated challengers and helped propel political allies into office.

Last year, Menino put that machine to work helping to elect Democratic U.S. Sen. Elizabeth Warren, who defeated incumbent Republican Sen. Scott Brown.

The machine was effective, but also ruthless, some said, often driving those who challenged or crossed the mayor to the sidelines. None of the mayor's opponents over the years would ever achieve much, if any, political success in the future.

Despite his political savvy, Menino was also known for his sometimes tortured phrases and malapropisms, which earned him the nickname "Mumbles" from detractors.

In a city known for die-hard sports fans, his occasionally comic gaffes when speaking about Boston sports stars only seemed to endear him even more to the populace.

He once confused former New England Patriots placekicker and Super Bowl hero Adam Vinatieri with former Red Sox catcher Jason Varitek. He called Celtics star Kevin Garnett "KJ" instead of Garnett's nickname "KG," and referred to point guard Rajon Rondo as "Hondo" which was the nickname of former Celtics great John Havlicek.